Strain Gage Connections

When designing and testing materials and products it is essential to understand its material properties and how it reacts to the forces and loads applied to it. The deformation a DUT experiences when a force is applied is called strain. Strain is the ratio of the change in length of a material to the original length. Strain can be positive (tensile) or negative (compressive).

Strain gages have long been the premier analytic tool for static stress/strain analysis, characterizing the basic strength of structures. The strain gage, a tiny nearly mass-less and very compliant device is often hailed as the perfect tool to measure strain. While this is true in sprit, it is essential to have the right tool for the right job or in our case the correct configuration to measure the correct type of strain. Strain can come in four different types: axial, bending, shear, and torsional.

Axial strain - Material stretches or compresses as a result of a linear force in the horizontal (along the axis) direction.

Bending strain - Material stretches on one side of a material and contracts on the opposite side due to the linear force applied in the vertical direction.

Shear strain - the amount of deformation that occurs from a linear force with components in both the horizontal and vertical directions.

Torsional strain - deformation due to a circular force with components in both the vertical and horizontal directions.

*DUTs in Axial (left) and Bending (right) Strain.*

Types of Strain Gages
The three types of strain gage configurations, quarter-, half-, and full-bridge, are determined by the number of active elements in the Wheatstone bridge, the orientation of the strain gages, and the type of strain being measured.

Quarter-Bridge Strain Gage
The quarter bridge configuration utilizes one strain gage connected to the EXC+ and In- inputs of a Spider-80SG channel. No other connections are required. Strain gages are not polarized, so it does not matter which side of the gage is connected to which input. The Spider-80SG provides the other three resistors to complete the bridge. During software setup, users specify the gage to be either a 120 or 350-ohm unit, and the Spider-80SG switches in a matching value resistor to complete the bridge.

Configuration Type I (Q1)

Measures axial or bending strain
Single Active gauge

Configuration Type II (Q2)

Measures axial or bending strain
1 active gauge and 1 dummy gauge
Has gauge temperature compensation

This type of setup is primarily used where strain gauge experiences thermal strain as well. This commonly occurs when the DUT is tested under running conditions or environmental settings where heat is generated, dissipated or cycled. This temperature gradient causes the gauge resistance to vary, so to compensate for this we add a second gauge (dummy). The “dummy” gauge is in the same temperature conditions as the strain gauge but is not physically connected to the DUT and does not experience any strain.

*Quarter Type 2 Circuit showing dummy resistance*

*Quarter-Bridge Strain Gage on DUT with dummy gauge*

Half-Bridge Strain Gage

Configuration Type I

Measures axial or bending strain
R4 is an active strain gage measuring the tensile strain (+ε)
R3 is an Poisson strain gage compensating for Poisson’s effect (-νε)

Configuration Type II

Measures bending strain only
R4 is an active strain gage measuring the tensile strain (+ε)
R3 is an active strain gage measuring the compressive strain (-ε)
2 active gauges leading to twice the sensitivity

*Half Bridge Circuit: Both configurations have the same connections*

Full-Bridge Strain Gage
A full-bridge strain gage configuration has four active strain gages, and these can be used in three different types. Types 1 and 2 measure bending strain and type 3 measures axial strain. Only types 2 and 3 compensate for the Poisson effect, but all three types minimize the effects of temperature.

Configuration Type I

Highly sensitive to bending strain only (4X) – 4 Active gauges
2 active strain gages measuring compressive strain (–e)
2 active strain gages measuring tensile strain (+e)

Configuration Type II

Sensitive to bending strain only
R1 is an active Poisson gage measuring the compressive Poisson effect (–νe)
R2 is an active Poison gage measuring the tensile Poisson effect (+νe)
R3 is an active strain gage measuring the compressive strain (–e)
R4 is an active strain gage measuring the tensile strain (+e)

Configuration Type III

Measures axial strain
2 active Poisson gages measuring the compressive Poisson effect (–νe)
2 are active strain gages measuring the tensile strain (+e)

  
    Measurement Type
    Quarter Bridge
    Half-Bridge
    Full-Bridge
  

    Type I
    Type II
    Type I
    Type II
    Type I
    Type II
    Type III
  

    Axial Strain
    Yes
    Yes
    Yes
    No
    No
    No
    Yes
  

    Bending Strain
    Yes
    Yes
    Yes
    Yes
    Yes
    Yes
    No
  

    Poisson compensation
    No
    No
    Yes
    No
    No
    Yes
    Yes
  

    Temperature Compensation
    No
    Yes
    Yes
    Yes
    Yes
    Yes
    Yes
  

For further information on strain gauges and strain measurement devices, please contact our team at Crystal Instruments or take a look at some of our other articles:

A New Approach to an Old Sensor – A Spider Tames the Strain Gage - by George Fox Lang
Vibration Control Using Strain Gages: New Strain Rate Feedback - by Vishaal Subramanian
Mounting Strain Gage on UUT - by Trung-Duong Nguyen
Breakout Box Terminal Guide for Strain Gage Measurement on Spider-80SG - by Trung-Duong Nguyen

DAQ (Strain/Temperature)Vishaal Subramanian - Aerospace Application EngineerMarch 27, 2020Machine Condition Monitoring, Structural Dynamics Testing